On January 1, a new state law goes into effect in California. It will limit the amount of contact in high school and middle school football practices. The state is hoping to protect young brains from football injuries. NPR's Tom Goldman reports.

TOM GOLDMAN, BYLINE: It's called the Oklahoma drill, but it's a football practice tradition everywhere. The Oklahoma drill pits a couple of offensive players against a couple of defenders. It captures the essence of full contact, which one Texas High School Athletic Director described as full game-speed, full 100 percent knocking the snot out of each other.

Coaches at all levels of the game have long used this drill to get players used impact after a long off-season, to see how players react to hitting and getting hit, to bond the team in that primal football kind of way. But starting next season, California coaches won't be able to use the Oklahoma drill any time they like, because full contact will be on the clock.

KEN COOLEY: No more than three hours in two different days - two 90 minute sessions.

GOLDMAN: That's per week, says California Assemblyman Ken Cooley. He was the driving force behind the bill - now law - that limits full contact in practice and forbids it during the off-season. Cooley was motivated by several factors. Brain trauma researchers at the Sports Legacy Institute say anywhere from half to three-quarters of high school football concussions happen in practice. Parents he met said they'd never let their kids play football, and those who did are staying vigilant.

Troy Spencer's son has played the last two years for Southern California football power Centennial High. Spencer, who's also a youth football coach, says he's watched nearly all of his son's practices at Centennial, a school that he says does it right when it comes to full contact. Still, he's glad there's a new law.

TROY SPENCER: I've always felt like sometimes there's too much contact in practice when it doesn't matter. I understand you have to learn how to tackle and learn how to practice. But I think it starts all the way at the youth level, so by the time they get to high school, I don't mind them toning down how much contact they have.

GOLDMAN: Neither does Centennial head coach Matt Logan. Although he does mind being told to do it.

MATT LOGAN: I kind of feel like that we should be able to govern that on our own, that we wouldn't need legislation to do that. I think as football coaches, we understand the brutality of the sport, and most of us played it.

GOLDMAN: But not at a time when concussions were front and center, prompting change right now. California is one of a number of states to limit full contact in practice, but there's not a lot of uniformity in the quest for safety. Last year, Texas put in place a 90-minute-per-week limit. California, remember, is 180 minutes. So are states just fishing for numbers?

GERARD JOYA: Well, you know, that is a great question.

GOLDMAN: But concussion expert Dr. Gerard Joya says the answer remains elusive because the science isn't there yet to indicate what the right number is for limiting full contact.

JOYA: We all believe that, you know, less is more. And the question is, where do we draw that line to balance the appropriate kinds of exposure? And we clearly are in search of that important information.

GOLDMAN: Even without it, a national high school concussion taskforce that included Joya, strongly recommended in July no more than 60 to 90 minutes of full contact per week. California's new law is a great first step forward, according to Roger Blake. He's the state's director of high school sports. He thinks most coaches support the plan and believe they can teach the game properly even with the contact limits. Coaches will self-monitor. Blake hopes the few who fudge on the law will hear from principals, parents and the players, who need to know, Blake says, that this is about the rest of their lives. Tom Goldman, NPR News. Transcript provided by NPR, Copyright NPR.